This invention relates to an inductive device for precisely positioning a movable member along a rectilinear path and, more particularly, to a device for precisely positioning a magnetic head relative to a magnetic disc in apparatus for the high density recording and reading of information.
Various means exist for furnishing the information necessary for the displacement and positioning of magnetic head transducers in disc memories. If the head-bearing carriage is driven by a rotating threaded shaft, the number of revolutions or fractions of revolutions completed by the shaft is measured by an arrangement, such as a disc, having openings disposed between a photo-detector and a light source. However, if a more flexible and reliable mode of propulsion is desired, with higher accelerations and velocities for the carriage on the one hand and a detection system having faster response times on the other hand, a ball bearing carriage is employed, for example. This carriage moves on rails and is driven either by a motor with a movable coil, by a linear motor, or by a linear hydraulic motor controlled by variable flow valves. In these instances, the detection devices utilized are inductive coupling devices and provide information as to the track of the magnetic disc on which the transducing heads are located. The more frequently utilized inductive coupling devices for this purpose are formed of two opposed flat conductors in the shape of rectangular-like windings. One of the two conductors is attached to the movable carriage and the other to the fixed portion of the chassis of the disc memory. One of these conductors plays the role of transmitter and the other the role of receiver, and the arrangement acts as a transformer. Such transformer has a coupling that varies according to the relative position of the transmitter and receiver between a maximum positive value when the crests of the windings oppose each other and a maximum negative value when the crests are opposite troughs.
If the transmitter is supplied with a relatively high frequency current, of the order of a few hundred of KHz, the receiver receives a voltage modulated in amplitude corresponding to the movement of the carriage. Each passage of the demodulated voltage through zero corresponds to a track.
However, such systems require relatively large currents, of the order of an ampere, in the transmitter, thereby posing problems of size and cooling of the conductor forming the winding. Moreover, since the signals received by the receiver are weak, chains of sufficiently strong amplification are required for the receiver as well as for the transmitter.
Accordingly, it is the object of the present invention to remedy these disadvantages of the prior art positioning devices.
Another object of the present invention is to provide an inductive device for precisely positioning a movable member along a rectilinear path, wherein said member may be, for example, a carriage bearing the writing and reading heads of a magnetic disc memory.